<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
  <script>
    var message;
    alert(message);//undefined

    alert(typeof message2);////undefined

    alert(message3); //报 ReferenceError: message3 is not defined 的错误

  </script>
</head>
<body>

</body>
</html>